You keep talking about fairness and goalposts as though this is a sport, but aikido isn't supposed to be fair. There are a number of issues that need to be addressed here.

1. Is your sensei taking cheap shots?

Well that really depends. Is it normal to show you your openings by striking? - yes it is. It is a much more effective teaching method to show you than to tell you. BUT, if your sensei is hurting you, then he is taking cheap shots. A little tap to show that you are open is all that is necessary.

2. Why are different rules applied to different people?

Again, this boils down to aikido not being a sport. In reality there are no rules. The point of training in a dojo is to improve your ability in a safe environment. This means that what is appropriate for people at some levels is not appropriate for people at other levels. So, if your problem is "why is sensei doing this to me, and not to other people?" the answer is, because he thinks you are ready for it, whereas other people may not be. If the question is, "why can sensei do this to people, but I can't" the answer is, because he is the sensei. He is the one who is responsible for teaching, not you.

The rule against getting creative as uke is usually a matter of utility and safety. There is no point getting creative and showing people their openings when they are still learning the basic movements. That impedes their learning rather than helping it. Equally, getting creative as uke requires a great deal of sensitivity and ability to protect yourself. If nage just happens to get the technique right and catches uke unawares that can be very dangerous, so the rule is also there to protect you.

Having said all of that, it is part of uke's role to help tori learn their techniques, and pointing out openings is part of that, as long as you are confident in your ukemi, and your training partner is of a fairly high level (around 1kyu, or shodan I guess). If your sensei is preventing you from helping your training partner in those circumstances, you might want to discuss it with him/her.